The present invention relates generally to a structure in which one or more planar optical waveguides are coupled to one or more photodiodes, each of which is an optical active device, in order to convert one or more optical signals flowing along optical paths, which configure an end of a Planar Lightwave Circuit (PLC) device, into electric signals.
Here, the planar light wave circuit device means an optical device for enabling one or more optical signals to flow through optical paths formed on a planar surface by implementing a structure the same as optical fibers, which transmit light without loss, and for performing functions of multiplexing the optical signals having various wavelengths, demultiplexing the multiplexed optical signals into optical signals each having separate wavelength, attenuating the power of the optical signals, splitting optical power, and switching optical paths.
An Arrayed Waveguide Grating (AWG) device is an example of a representative planar light wave circuit device used to multiplex optical signals having various wavelengths and to demultiplex the multiplexed optical signals into optical signals each having a separate wavelength in an optical communication field.
Generally, an AWG device, which is a PLC device, performs a demultiplexing function of outputting a plurality of multiplexed optical signals having various wavelengths, input through a single input optical waveguide, through a plurality of output optical waveguides, and performs a multiplexing function of outputting a plurality of signals having various wavelengths, which are input through a plurality of input optical waveguides, through a single output optical waveguide.
A device, which adjusts the optical signals, is called an optical passive device, and is formed of silica medium, the refractive indexes of which are different from each other, on a silicon substrate. The AWG device is generally manufactured through a process of layering a cladding layer and a core layer on a substrate, performing etching on the core layer through a lithography process and a dry etching process and forming an optical path, through which an optical signal flows, along the core patterned in various forms, and layering the cladding layer again on the substrate on which the patterned core is formed. The planar optical waveguide which configures the tap coupler of FIG. 5 is manufactured using the above-described process.
When an optical-sub-system for processing optical signals by integrating PLC devices, such as the AWG device, a multi-port Variable Optical Attenuator (VOA) and an optical power splitter, is configured, it is preferred that the power of optical signals, input to/output from the respective input ports/output ports of each of the PLC devices, each having a plurality of input ports and a plurality of output ports, be monitored and then be uniformly adjusted.
Here, in order to monitor the optical signals of the respective input/output ports, it is required to install one or more tap couplers on one or more input/output optical waveguides, connected to the one or more input/output ports, to branch one or more optical signals into one or more optical waveguides, which are manufactured using the tap couplers, and to monitor the power of the branched optical signals using one or more photodiodes, which are optical active devices.
Such a photodiode, used in this case, is a representative optical active device and performs a function of switching an optical signal into an electric signal. A razor diode which performs a function of switching an electric signal into an optical signal may be another example of the optical active device. In order to handle optical signals having wavelengths in the range of 1310-1550 nm, which is mainly used in optical communication, using a photoelectric effect or an electro-optic effect, a p-n junction layer is formed by layering InGaAs materials, the generation rates of which differ from each other, on an InP substrate, thereby manufacturing an optical active device capable of switching an optical signal into an electric signal or switching an electric signal into an optical signal.
When an optical active device is coupled to an optical passive device for use, the optical passive device and the optical active device are formed of different materials, and thus the optical passive device and the optical active device cannot be manufactured on a single substrate using the same process. Therefore, the respective devices, which are completed using their respective processes, should be aligned and then attached. The operation of coupling an optical active device and an optical passive device, which are made of different materials and then integrated, is called hybrid integration.
Prior art hybrid integrated technology discloses a method of carving one or more trenches, each of which is narrow and inclined, so as to cut one or more planar optical waveguides which configure a PLC device, inserting one or more reflection filters so as to reflect optical signals, which flow through the planar optical waveguides, outside the cores of the respective planar optical waveguides, and then enabling the optical signals to be incident on the light receiving regions of one or more photodiodes. In this case, in order to attach one or more optical active devices to one or more optical passive devices, silicon platforms are formed on the substrates of the respective optical passive devices, one or more planar optical waveguides and the optical active regions of the respective optical active devices are accurately aligned, and then packaging is performed through flip chip bonding. FIGS. 1 and 2 are views showing a structure to which a PLC device 40 and a photodiode device 50, which is an optical active device, are coupled according to the prior art.
First, a PLC device module that can be actually used in a worksite is configured to include an input optical fiber array, to which an optical connector, that is, an incidence port, is attached, an output optical fiber array, to which an optical connector, that is an emission port, is attached, and a PLC device interposed therebetween and configured to adjust optical signals (demultiplex optical signals, multiplex optical signals, and mediate optical power). Furthermore, in order to switch an optical signal flowing through a planar optical waveguide into an electric signal, the PLC device module should further include a photodiode, which is an optical active device, and an electric circuit for connecting the photodiode. Here, the photodiode is a device for outputting current or voltage, which is an electric signal proportional to received optical power.
In this case, a prior art structure is configured such that a trench 35, having a predetermined angle inclined in a depth direction, is carved in one end of an output optical waveguide so as to cut the core 12 of a planar optical waveguide, a reflection mirror 11 having predetermined reflectance is inserted therein, an optical signal flowing through the core 12 of the planar optical waveguide is reflected at a predetermined angle, and reflected light 17 is received by the light reception region 51 of a photodiode placed on the end of the path of the reflected light. Here, if the reflectance of the reflection mirror 11, having predetermined reflectance, is adjusted, some or all of the light can be received by the photodiode 50.
Further, the cut surface of the trench 35 formed on an end of the output optical waveguide should be very clean so as to prevent light from being dispersed, and the trench 35 should be almost as narrow as the reflection mirror 11 to be inserted, so that the thin reflection mirror 11 can be accurately placed without being tilted, thereby uniformly maintaining a reflection angle. Further, the transmittance of the reflection mirror 11 is adjusted, and the thickness thereof is less than several tens of micrometers, such that an optical signal can be transmitted to an optical waveguide connected behind the reflection mirror 11 without loss.
Further, as long as the angle of the trench 35 is accurate when the trench 35 is formed, the reflected optical signal does not deviate from the light reception region 51 of the photodiode. Therefore, it is very difficult to set the location and shape of the trench 35 and the structure and thickness of the reflection mirror 11 such that they satisfy the above characteristics, and to insert the reflection mirror 11 within the trench 35.
Furthermore, in a planar optical waveguide structure, it is very difficult to manufacture a trench which satisfies the above characteristics using an etching (wet etching or dry etching) method or a sawing method.
Further, the reflection mirror 11 that is used has a thickness of 100 μm or less, and has a multi-layered thin film structure in which thin layers (thin films) having different refractive indexes are alternately layered. Therefore, in order to obtain desired reflectance, the reflection mirror 11 must be handled such that the surface of the reflection mirror 11 is not damaged when the reflection mirror 11 is inserted, and the reflection mirror 11 must be accurately inserted into and fixed within the trench 35, such that the reflection mirror 11 has a predetermined angle within the trench 35. Accordingly, there is a problem in that errors occur in the location of the reflection mirror 11 due to the irregularity generated when the trench 35 is substantially formed and the irregularity generated when the reflection mirror 11 is inserted.
Furthermore, there are problems in that it is difficult to form a trench, which is narrow and inclined in a depth direction and to which a reflection mirror will be inserted, at an accurate location, and in that an optical signal to be reflected is not accurately incident on the light reception region of a photodiode due to a tilt phenomenon occurring when the reflection mirror is inserted into the trench and fixed using a refractive index matching epoxy.
FIGS. 3 and 4 show another prior art design disclosing a structure in which reflection and transmission are simultaneously realized by grinding one end of the output optical waveguide of a PLC device 40 to be inclined, and then using the ground surface thereof without change or inserting/attaching a multi-layered thin film filter.
That is, instead of carving a trench, which is difficult to handle, on a planar optical waveguide, a structure is configured such that an end of the output optical waveguide of a PLC device and the input end 30 of an output optical fiber array are ground to be inclined along an optical axis, a multi-layered thin film filter 11 capable of adjusting reflectance and transmittance is attached at a boundary surface between the output optical waveguide and the output optical fiber array, and part of the power of the optical signal flowing through the planar optical waveguide is reflected at a predetermined angle, so that reflected light 17 is received by the light reception region 51 of a photodiode 50 placed on an end of an optical path, and remaining power of the optical signal is output through the output optical fiber array.
In this case, since a reflection surface is formed on each of the ends of all the output optical waveguides existing in the output end of the PLC device 40 due to a multi-layered thin film filter 11, the reflection surface is applied to one or more output optical waveguides which are not desired to be monitored, thereby causing additional optical loss.
In particular, when the number of ports of the PLC device 40 increases, the area in which output optical waveguides are arranged increases. Therefore, the size of the multi-layered thin film filter 11 must increase. Further, as the size of the multi-layered thin film filter 11 increases, the thickness of the substrate for supporting the multi-layered thin film of a multi-layered thin film filter 11 increases. Therefore, as shown in FIG. 4, since the interval between the output optical waveguide and the output optical fiber array is wide, optical signals, which pass through the multi-layered thin film filter 11 and are then output through the optical fiber array, suffer additional loss.
Further, since the multi-layered thin film filter 11 is used for a part which does not require an expensive multi-layered thin film filter 11, that is, an empty part provided to maintain a predetermined interval between output optical waveguides, the cost of the multi-layered thin film filter 11 is increased.
Further, it is difficult to perform a process of uniformly attaching a multi-layered thin film filter on the output surface of the PLC device 40 or the input surface of the optical fiber array 30 without forming an air layer or flexure.
Furthermore, the air layer and flexure disturb the alignment and bonding of an output optical waveguide and an output optical fiber array, and affect the optical path, thereby causing an optical signal coupled to the optical fiber array to have additional loss.
Further, since it is difficult to manufacture a multi-layered thin film filter having the same reflectance with respect to all optical signals having various wavelengths, there is a problem in that it is difficult to apply the multi-layered thin film filter to a PLC device for handling a wide wavelength band.